Aminoacyl tRNA synthetase regulation. One form of the valyl tRNA synthetase of yeast can be dissociated with separation of a heat-stable, nondialyzable acidic substance that stimulates the enzyme's action. The heat-stable material has now been separated into at least four fractions, one of which has been purified to homogeneity. It is a peptide of about 18,000 molecular weight and appears to contain two residues of half cystine per mole. Evolution. The capacity of nucleotide bases to form hydrogen bonds with peptides has been further investigted with the interaction of cytosine with peptides of serine and threonine the central point of interest. This interaction, previously demonstrated in organic solvents, has now been shown to occur also in water when suitable derivatives are used.